Single molecule detection from a large-scale SERS-active Au79Ag21 substrate

Detecting and identifying single molecules are the ultimate goal of analytic sensitivity. Single molecule detection by surface-enhanced Raman scattering (SM-SERS) depends predominantly on SERS-active metal substrates that are usually colloidal silver fractal clusters. However, the high chemical reactivity of silver and the low reproducibility of its complicated synthesis with fractal clusters have been serious obstacles to practical applications of SERS, particularly for probing single biomolecules in extensive physiological environments. Here we report a large-scale, free standing and chemically stable SERS substrate for both resonant and nonresonant single molecule detection. Our robust substrate is made from wrinkled nanoporous Au79Ag21 films that contain a high number of electromagnetic “hot spots” with a local SERS enhancement larger than 109. This biocompatible gold-based SERS substrate with superior reproducibility, excellent chemical stability and facile synthesis promises to be an ideal candidate for a wide range of applications in life science and environment protection

Dynamic behaviour of interphases and its implication on high-energy-density cathode materials in lithium-ion batteries

Undesired electrode-electrolyte interactions prevent the use of many high-energy-density cathode materials in practical lithium-ion batteries. Efforts to address their limited service life have predominantly focused on the active electrode materials and electrolytes. Here an advanced three-dimensional chemical and imaging analysis on a model material, the nickel-rich layered lithium transition-metal oxide, reveals the dynamic behaviour of cathode interphases driven by conductive carbon additives (carbon black) in a common nonaqueous electrolyte. Region-of-interest sensitive secondary-ion mass spectrometry shows that a cathode-electrolyte interphase, initially formed on carbon black with no electrochemical bias applied, readily passivates the cathode particles through mutual exchange of surface species. By tuning the interphase thickness, we demonstrate its robustness in suppressing the deterioration of the electrode/electrolyte interface during high-voltage cell operation. Our results provide insights on the formation and evolution of cathode interphases, facilitating development of in situ surface protection on high-energy-density cathode materials in lithium-based batteries.ope

Direct Experimental Probe of the Ni(II)/Ni(III)/Ni(IV) Redox Evolution in LiNi0.5Mn1.5O4 Electrodes

The LiNi0.5Mn1.5O4 spinel is an appealing cathode material for next generation rechargeable Li-ion batteries due to its high operating voltage of ∼4.7 V (vs Li/Li+). Although it is widely believed that the full range of electrochemical cycling involves the redox of Ni(II)/(IV), it has not been experimentally clarified whether Ni(III) exists as the intermediate state or a double-electron transfer takes place. Here, combined with theoretical calculations, we show unambiguous spectroscopic evidence of the Ni(III) state when the LiNi0.5Mn1.5O4 electrode is half charged. This provides a direct verification of single-electron-transfer reactions in LiNi0.5Mn1.5O4 upon cycling, namely, from Ni(II) to Ni(III), then to Ni(IV). Additionally, by virtue of its surface sensitivity, soft X-ray absorption spectroscopy also reveals the electrochemically inactive Ni2+ and Mn2+ phases on the electrode surface. Our work provides the long-awaited clarification of the single-electron transfer mechanism in LiNi0.5Mn1.5O4 electrodes. Furthermore, the experimental results serve as a benchmark for further spectroscopic characterizations of Ni-based battery electrodes

Integrated nano-domains of disordered and ordered spinel phases in LiNi0.5Mn1.5O4 for li-ion batteries

Recent calculations and experimental data suggest that understanding the local ordering behavior of Ni/Mn will be critical to optimize the electrochemical properties of LiNi Mn O (LNMO) high voltage spinel. In this study, we systematically controlled the evolution of Ni and Mn ordering in LNMO samples by annealing them at 700 °C in air for different dwelling times, followed by quenching to room temperature. The long- and short-range ordering behavior of Ni and Mn were analyzed by combining neutron powder diffraction, X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FT-IR) data. The results show that the fraction of ordered phase increases rapidly during initial annealing at 700 °C for 6 h, and accompanied by decreasing amounts of secondary phases. Annealing longer than 6 h led to the growth in size of ordered domains (i.e., increased segregation of ordered and disordered domains) along with a slow increase in the fraction of ordered phase. The dependence of open circuit voltages (OCVs) of the LNMO on the degree of ordering agrees well with recent calculations using the density functional theory. The increase in the degree of ordering increases the open circuit voltage (OCV) and the initial capacity but reduces cycle life and rate capability. The LNMO delivered optimal battery performances (capacity, cycle life, and rate capability) after annealing at 700 °C for 2 h. This partially ordered sample showed the respective advantages from both disordered and ordered spinels: better spinel-phase purity (thus, higher initial capacity) from the ordered LNMO and better cycle life and rate capability from the disordered LNMO. © 2014 American Chemical Society. 0.5 1.5

Integrated nano-domains of disordered and ordered spinel phases in LiNi0.5Mn1.5O4 for li-ion batteries

Recent calculations and experimental data suggest that understanding the local ordering behavior of Ni/Mn will be critical to optimize the electrochemical properties of LiNi0.5Mn1.5O4 (LNMO) high voltage spinel. In this study, we systematically controlled the evolution of Ni and Mn ordering in LNMO samples by annealing them at 700 °C in air for different dwelling times, followed by quenching to room temperature. The long- and short-range ordering behavior of Ni and Mn were analyzed by combining neutron powder diffraction, X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FT-IR) data. The results show that the fraction of ordered phase increases rapidly during initial annealing at 700 °C for 6 h, and accompanied by decreasing amounts of secondary phases. Annealing longer than 6 h led to the growth in size of ordered domains (i.e., increased segregation of ordered and disordered domains) along with a slow increase in the fraction of ordered phase. The dependence of open circuit voltages (OCVs) of the LNMO on the degree of ordering agrees well with recent calculations using the density functional theory. The increase in the degree of ordering increases the open circuit voltage (OCV) and the initial capacity but reduces cycle life and rate capability. The LNMO delivered optimal battery performances (capacity, cycle life, and rate capability) after annealing at 700 °C for 2 h. This partially ordered sample showed the respective advantages from both disordered and ordered spinels: better spinel-phase purity (thus, higher initial capacity) from the ordered LNMO and better cycle life and rate capability from the disordered LNMO. © 2014 American Chemical Society